1. Field of the Invention
The present invention relates to compensation of data signals, and in particular, to compensation for signal crosstalk products within data signals that have been multiplexed and conveyed by a signal transmission medium, wherein such signal crosstalk products are related to one or more interactions among the multiplexed data signals within the signal transmission medium.
2. Description of the Related Art
Referring to FIG. 1, a conventional fiber optic signal system 10 often uses wavelength-division multiplexing (WDM) to increase the signal capacity of the fiber optic signal transmission medium. Accordingly, such a system 10 will typically include multiple optical signal transmitters 12, an optical multiplexor 14, the fiber optic medium 16, an optical demultiplexor 18 and multiple optical signal receivers 20, all interconnected substantially as shown.
As is well known in the art, electrical data signals 11 are converted by the optical signal transmitters 12 to optical signals 13, which are then multiplexed by the muliplexor 14 to provide the multiplexed signal 15 containing all of the optical channels at the various wavelengths λ1, λ2, λ3, . . . , λn. This multiplexed signal 15 is then conveyed by the fiber optic signal transmission medium 16. At the end of the fiber optic signal path 16 the received signal 17, which will have a number of signal crosstalk products (discussed in more detail below), is demultiplexed by the demultiplexor 18. The resulting individual optical signals 19 are then converted by the optical signal receivers 20 to corresponding electrical data signals 21.
To varying degrees, each of the demultiplexed optical data signals 19a, 19b, 19c, . . . , 19n will include one or more signal crosstalk products related to one or more interactions among these signals during their conveyance as a single multiplexed signal through the fiber optic signal transmission medium 16 (discussed in more detail below). These signal crosstalk products generally remain (and may become worse) and become part of the corresponding electrical data signals 21a, 21b, 21c, . . . , 21n. Such signal crosstalk products can be generated by a number of well known signal interactions that often take place within a signal transmission medium such as an optical fiber, and include those caused by dense wavelength-division multiplexing (DWDM), four-wave mixing (FWM) and cross-phase modulation (XPM).
In the case of DWDM, signal interactions increase as the channel spacing between the optical signals decreases. As is well known, channel density is a key parameter in WDM systems. An international standard specifies standard center frequencies to be separated by 100 gigahertz (GHz), corresponding to approximately 0.8 nanometers in an erbium-fiber amplifier band. Some commercial optics systems use frequency spacing on a 50 GHz grid. Further developments may reduce channel spacing to 25 GHz or perhaps even 12.5 GHz. In any event, as channel spacing becomes more dense, the likelihood and degree to which signal interactions take place increase significantly.
Referring to FIG. 2, one well known nonlinear effect in WD systems is that of FWM in which three input frequencies interact by combining within the signal transmission medium to generate a mixed signal at a fourth frequency. For example, the signals at frequencies υ1, υ2, υ3 may interact or combine in such a manner that the signals at υ1 and υ2 are summed while the signal at υ3 is subtracted in frequency, thereby producing a signal at υ4. (It should be noted that the three input signals υ1, υ2, υ3 need not be at their own respective frequencies; two of them could be on the same optical channel.) Equal spacing of WDM channels can cause the newly generated signal υ4 to fall within another optical signal channel, thereby producing noise and crosstalk that interferes with the original signal on that channel. The amount of FWM is proportional to the length of the transmission medium over which such signal interactions take place. While such nonlinear signal interactions tend to be relatively weak in glass fiber, the strength of these interactions accumulate with distance.
Another nonlinear effect is that of XPM in which variations in the intensity of one optical signal channel can cause changes in the refractive index of the fiber optic medium, thereby affecting other optical signal channels. Such changes in the refractive index modulate the phase of the light within the other optical signal channels (as well as increase self-phase modulation of the reference channel, i.e., that channel causing such change in the refractive index). The strength of XPM effects increases with the number of optical signal channels, and increases further as the channel spacing becomes more dense.